Effect of Metal Bond-Pad Configurations on the Solder Microstructure Development of Flip-Chip Solder Joints

Various microstructural zones were observed in the solidified solder of flip-chip solder joints with three metal bond-pad configurations (Cu/Sn/Cu, Ni/Sn/Cu, and Cu/Sn/Ni). The developed microstructures of the solidified flip-chip solder joints were strongly related to the associated metal bond pad. A hypoeutectic microstructure always developed near the Ni bond pad, and a eutectic or hypereutectic microstructure formed near the Cu pad. The effect of the metal bond pads on the solder microstructure alters the Cu solubility in the molten solder. The Cu content (solubility) in the molten Sn(Cu) solder eventually leads to the development of particular microstructures. In addition to the effect of the associated metal bond pads, the developed microstructure of the flip-chip solder joint depends on the configuration of the metal bond pads. A hypereutectic microstructure formed near the bottom Cu pad, and a eutectic microstructure formed near the top Cu pad. Directional cooling in the flip-chip solder joint during the solidification process causes the effects of the metal bond-pad configuration. Directional cooling causes the Cu content to vary in the liquid Sn(Cu) phase, resulting in the formation of distinct microstructural zones in the developed microstructure of the flip-chip solder joint.     

Effect of surface finish on the failure mechanisms of flip-chip solder joints under electromigration

Two substrate surface finishes, Au/Ni and organic solderable preservative (OSP), were used to study the effect of the surface finish on the reliability of flip-chip solder joints under electromigration at 150°C ambient temperature. The solder used was eutectic PbSn, and the applied current density was 5×10³ A/cm² at the contact window of the chip. The under bump metallurgy (UBM) on the chip was sputtered Cu/Ni. It was found that the mean-time-to-failure (MTTF) of the OSP joints was six times better than that of the Au/Ni joints (3080 h vs. 500 h). Microstructure examinations uncovered that the combined effect of current crowding and the accompanying local Joule heating accelerated the local Ni UBM consumption near the point of electron entrance. Once Ni was depleted at a certain region, this region became nonconductive, and the flow of the electrons was diverted to the neighboring region. This neighboring region then became the place where electrons entered the joint, and the local Ni UBM consumption was accelerated. This process repeated itself, and the Ni-depleted region extended further on, creating an ever-larger nonconductive region. The solder joint eventually, failed when the nonconductive region became too large, making the effective current density very high. Accordingly, the key factor determining the MTTF was the Ni consumption rate. The joints with the OSP surface finish had a longer MTTF because Cu released from the substrate was able to reduce the Ni consumption rate.     

Geometrical effect of bump resistance for flip-chip solder joints: Finite-element modeling and experimental results

The bump resistance of flip-chip solder joints was measured experimentally and analyzed by the finite-element method. Kelvin structures for flip-chip solder joints were designed and fabricated to measure the bump resistance. The measured value was only about 0.9 mΘ at room temperature, which was much lower than that expected. Three-dimensional (3-D) modeling was performed to examine the current and voltage distribution in the joint. The simulated value was 7.7 mΘ, which was about 9 times larger than the experimental value. The current crowding effect was found to be responsible for the difference in bump resistance. Therefore, the measured bump resistance strongly depended on the layout of the Kelvin structure. Various layouts were simulated to investigate the geometrical effect of bump resistance, and a significant geometrical effect was found. A proper layout was proposed to measure the bump resistance correctly. The Kelvin structure would play an important role in monitoring void formation and microstructure changes during the electromigration of flip-chip solder joints.     

In-situ observation of material migration in flip-chip solder joints under current stressing

This investigation studies how electron flow distribution and the vacancy concentration gradient affect the diffusion of solder atoms in a flip-chip solder joint under current stress. The migration of materials was traced by monitoring the positions of 21 Pb grains of the eutectic PbSn solder joint. Experimental results indicate that the displacements of the Pb grains were not uniform along the electron flow direction. Additionally, certain Pb grains exhibited lateral displacements. The nonuniform material migration is attributable to the combined effect of electromigration and the vacancy concentration gradient, which was caused by electromigration. By measuring the displacements of the Pb grains, we estimated that the DZ* value of Sn in eutectic SnPb solder was 5×10⁻¹⁰ cm²/s.     

Interconnections based on Bi-coated SnAg solder balls

To decrease the bonding temperature required for eutectic SnAg solder, SnAg solder bumps were chemically coated with a pure Bi layer. During heating, a low melting eutectic forms between the Bi coating and the SnAg, enabling bonding at temperatures below the melting points of either pure Bi or SnAg solder. As the composition of the molten solder changes toward more dilute Bi concentrations, the melting point in the joint region increases and the joint solidifies. After solidification the joints will no longer melt at the original bonding temperature. Bi-coated SnAg solder balls were joined to metallized substrates at temperatures ranging from 180°C to 250°C. The microstructure at the joint interface was characterized by the SEM/EDS technique. As expected, at 180°C the Bi-coated SnAg solder balls melted only locally at the interfacial regions between the ball and the substrate and so retained their spherical shape during bonding. After solidification there were a lot of small Bi precipitates in the joint region. At higher temperatures, the wetting was evidently better, and there were less Bi precipitates, because the melt was more dilute in bismuth. In all cases, Bi formed relatively small, equi-axed precipitates instead of the eutectic structure found in eutectic Sn-Bi solder joints     

Effect of temperature cycling on joint strength of PbSn and AuSnsolders in laser packages

The effect of temperature cycle testing on the joint strength of PbSn and AuSn solders in laser diode packages has been studied experimentally and numerically. Experimental results showed that the joint strength increased as the temperature cycle number increased initially, and then became steady after 400 cycles. The joint strengths of PbSn and AuSn solders increased about 40% to 20% after undergoing 500 temperature cycles, respectively. A finite-element method (FEM) analysis was performed on the calculation of joint strength variation of PbSn and AuSn solders in temperature cycling tests. The coupled thermal-elasticity-plasticity model was employed in the solidification and residual stresses calculation. Simulation results were in good agreement with the experimental measurements that the solder joint strength increased as the temperature cycle increased. Numerical results indicate that the increasing solder joint strength comes from the redistribution of the residual stresses within the solder during temperature cycling tests. The local yielding and the creep effects on the low melting temperature solders will make uniform the residual stresses distribution introduced in the solidification process and increasing the solder joint strength as the temperature cycle number increased. The result suggests that the FEM is an effective method for analyzing and predicting the solder joint strength in laser diode packages     

Electromigration-induced failure in flip-chip solder joints

The electromigration failure mechanism in flip-chip solder joints through the rapid dissolution of the Cu metallization was studied in detail. The ambient temperature was found to be a very important factor in this failure mechanism. When the ambient temperature was changed from 100°C to 70°C, the time to failure changed from 95 min to 31 days. The results of this study indicate that temperature, as an experimental variable, is not less important than the current density in electromigration study. The surface temperatures of the chip and substrate during electromigration were also measured. The temperature of the Si chip was reasonably homogeneous because of the fact that Si is a very good thermal conductor. It was also reasoned that the high thermal conductivity of the PbSn solder could not support a temperature gradient large enough to induce thermomigration across the solder joint in the present study. Experimentally, no evidence of mass transport caused by thermomigration was observed.     

The effect of soldering process variables on the microstructure and mechanical properties of eutectic Sn-Ag/Cu solder joints

Fundamental understanding of the relationship among process, microstructure, and mechanical properties is essential to solder alloy design, soldering process development, and joint reliability prediction and optimization. This research focused on the process-structure-property relationship in eutectic Sn-Ag/Cu solder joints. As a Pb-free alternative, eutectic Sn-Ag solder offers enhanced mechanical properties, good wettability on Cu and Cu alloys, and the potential for a broader range of application compared to eutectic Sn-Pb solder. The relationship between soldering process parameters (soldering temperature, reflow time, and cooling rate) and joint microstructure was studied systemati-cally. Microhardness, tensile shear strength, and shear creep strength were measured and the relationship between the joint microstructures and mechani-cal properties was determined. Based on these results, low soldering tempera-tures, fast cooling rates, and short reflow times are suggested for producing joints with the best shear strength, ductility, and creep resistance.     

Retarding Electromigration in Lead-Free Solder Joints by Alloying and Composite Approaches

The voids induced by electromigration (EM) can trigger serious failure across the entire cathode interface of solder joints. In this study, alloying and composite approaches showed great potential for inhibiting EM in lead-free solder joints. Microsized Ni, Co, and Sb particles were added to the solder matrix. Cu and Sn particles were added to the melting solder to form in situ Cu₆Sn₅, which formed a barrier layer in the underbump metallization of flip-chip solder joints. The polarity effect induced by EM was observed to be significantly inhibited in the alloyed and composite solder joints. This indicates that the Sn-Ni, Sn-Co, Sn-Sb, and Cu₆Sn₅ intermetallic compounds may act as barriers to obstruct the movement of the dominant diffusion species along phase boundaries, which in turn improves the resistance to EM. However, Sb particles could induce crack formation and propagation that might lead to joint fracture.     

Processing and aging characteristics of eutectic Sn-3.5Ag solder reinforced with mechanically incorporated Ni particles

Composite solders offer improved properties compared to non-composite solders. Ni reinforced composite solder was prepared by mechanically dispersing 15 vol.% of Ni particles into eutectic Sn-3.5Ag solder paste. The average size of the Ni particle reinforcements was approximately 5 microns. The morphology, size and distribution of the reinforcing phase were characterized metallographically. Solid-state isothermal aging study was performed on small realistic size solder joints to study the formation and growth of the intermetallic (IM) layers at Ni reinforcement/solder and Cu substrate/solder interfaces. Effects of reflow on microstructure and solderability, were studied using Cu substrates. Regarding solderability, the wetting angle of multiple reflowed Ni reinforced composite solder was compared to the solder matrix alloy, eutectic Sn-3.5Ag. General findings of this study revealed that Ni particle reinforced composite solder has comparable wetting characteristics to eutectic Sn-3.5Ag solder. Significant IM layers growth was observed in the Ni composite solder joint under isothermal aging at 150 C. Microstructural evolution was insignificant when aging temperature was lower than 100 C. Multiple reflow did not significantly change the microstructure in Ni composite solder joint.     

Development of a solder material process to relieve the plastic constraint associated with thin joints

High aspect ratio (large diameter/thickness) solder joints which are plastically constrained develop large hydrostatic stresses (Friction Hill) greatly in excess of their yield strength. Because the local high triaxial stresses arising from the Friction Hill prevent homogeneous yielding and, in a strain controlled system, will localize plastic deformation within the regions near free surfaces, abrupt brittle fracture through an intermetallic or along an interface can occur. In such situations, the service life of the joint during fatigue situations such as thermal cycling will be greatly reduced. The prevention of triaxial stress build up within such a strain controlled environment which can occur in, for example, leadless chip carrier solder joints requires a distribution of internal free surfaces within the joint. The solder system developed in this study is a thin porous metal film with a regular distribution of pores. The solder material is formed from the usual components, tin and lead. Small lead or tin particles are coated with a thin film of the other component, mixed with flux paste, and the temperature is raised to just above the eutectic temperature. Solid state diffusion occurs across the lead-tin interface until its composition reaches the melting point. The particles then are interconnected by a thin near eutectic liquid film. Additional metal from the solid particle dissolves into the liquid increasing its position and, thus its melting point. Diffusion into the liquid continues until it solidifies isothermally. This forms an interconnecting network of solder “mini-elements” with a dense pore structure.     

Microstructure evolution of eutectic Sn-Ag solder joints

Laser and infrared reflow soldering methods were used to make Sn-Ag eutectic solder joints for surface-mount components on printed wiring boards. The microstructures of the joints were evaluated and related to process parameters. Aging tests were conducted on these joints for times up to 300 days and at temperature up to 190°C. The evolution of microstructure during aging was examined. The results showed that Sn-Ag solder microstructure is unstable at high temperature, and microstructural evolution can cause solder joint failure. Cu-Sn intermetallics in the solder and at copper-solder interfaces played an important role in both the microstructure evolution and failure of solder joints. Void and crack formation in the aged joints was also observed.     

Fiber alignment shift formation mechanisms of fiber-solder-ferrulejoints in laser module packaging

The fiber alignment shifts of fiber-solder-ferrule (FSF) joints in laser module packaging under temperature cycle testing using PbSn and AuSn solders are studied experimentally and numerically. The measured results showed that the fiber shifts of FSP joints with the hard AuSn solder exhibited shifts two times less than that with the soft PbSn solder. This suggests that the hard solder may be more suitable for FSF assembly than the soft solder. The results also showed that fiber shifts increased as the temperature cycle number and the initial fiber eccentric offset increased. The experimental measurements of fiber shifts were in good agreement with the numerical calculations of the finite-element method analysis. The major fiber shift formation mechanisms of FSF joints in temperature cycling may come from the localized plastic solder yielding introduced by the local thermal stress variation, the redistribution of the residual stresses, and the stress relaxation of the creep deformation within the solder. Furthermore, the stress relaxation of creep deformation in solder with either 21% (PbSn solder) or 5% (AuSn solder) may have significant influence on the fiber shifts. This study has provided an optimum approach for reduction of the fiber alignment shift of FSF joints in laser module packaging under temperature cycle testing, which is to solder the fiber near to the center of the ferrule and to select the AuSn hard solder     

Effects of Forming Processes on the Microstructure and Solderability of Sn-3.5Ag Eutectic Solder Ribbons as well as the Mechanical Properties of Solder Joints

Two kinds of Sn-3.5Ag eutectic solder ribbons of 0.13 mm thickness were prepared by a casting–rolling process and a rapid solidification process. The microstructure, phase constitution, melting characteristics, wetting behavior and soldering strength were compared using optical microscopy, scanning electron microscopy, x-ray diffraction, energy dispersive spectroscopy, differential scanning calorimetry and a MTS ceramic testing system. The results show that the microstructure of rapidly solidified solder is finer and more uniform, and the eutectic structure has a higher solid solubility and more homogeneous distribution of Ag in a Sn matrix. The solidus and liquidus temperature decreased, resulting in a 3.3% reduction of pasty range. In addition, the wettability and shear strength of the solder joints increased by 13.2% and 7.9%, respectively.     

Mechanical response of solder joints in flip-chip type structures

The mechanical response of PbSn solder joints of two different solder alloys (37 wt.% Pb - 63 wt.% Sn and 95 wt.% Pb - 5 wt.% Sn) used as flip-chip type interconnects is measured through mechanical testing (in tension and in shear). The influence of solder pad composition (Au and Ni) upon the behaviour of the solder joints is examined. Fatigue testing performed upon flipchip samples demonstrates the difference in mechanical comportment between Pb37Sn63 and Pb95Sn5 solders. A model for predicting fatigue life is put forward.     

A new COG technique using low temperature solder bumps for LCD driver IC packaging applications

A new chip on glass (COG) technique using flip chip solder joining technology has been developed for excellent resolution and high quality liquid crystal display (LCD) panels. The flip chip solder joining technology has several advantages over the anisotropic conductive film (ACF) bonding technology: finer pitch capability, better electrical performance, and easier reworkability. Conventional solders such as eutectic Pb-Sn and Pb-5Sn require high temperature processing which can lead to degradation of the liquid crystal or the color filter in LCD modules. Thus it is desirable to develop a low temperature process below 160/spl deg/C using solders with low melting temperatures for this application. In our case, we used eutectic 58 wt%Bi-42 wt%Sn solder for this purpose. Using the eutectic Bi-Sn solder bumps of 50-80/spl mu/m pitch sizes, an ultrafine interconnection between the IC and glass substrate was successfully made at or below 160/spl deg/C. The average contact resistance of the Bi-Sn solder joints was 19m/spl Omega/ per bump, which is much lower than the contact resistance of conventional ACF bonding technologies. The contact resistance of the underfilled Bi-Sn solder joints did not change during a hot humidity test. We demonstrate that the COG technique using low temperature solder joints can be applied to advanced LCDs that lead to require excellent quality, high resolution, and low power consumption.     

Failure study of Sn37Pb PBGA solder joints using temperature cycling,random vibration and combined temperature cycling and random vibration tests

In this paper, the tin-lead (Sn-37wt%Pb) eutectic solder joints of plastic ball grid array (PBGA) assemblies are tested using temperature cycling, random vibrations, and combined temperature cycling and vibration loading conditions. The fatigue lives, failure modes for the solder joints and the typical locations of the failed solder joints for single-variable loading and combined loading conditions are compared and analyzed. The results show much earlier solder joint failure for combined loading than that for either temperature cycling or pure vibration loading at room temperature. The primary failure mode is cracking within the bulk solder under temperature cycling, whereas the crack propagation path is along the intermetallic compound (IMC) layer for vibration loading. The solder joints subjected to combined loading exhibit both types of failure modes observed for temperature cycling and vibration loading; in addition, cracking through the IMC and the bulk solder is observed in the combined test. For temperature cycling and vibration loading, the components in the central region of the printed circuit board (PCB) have more failed solder joints than other components, whereas for combined loading, the number of failed solder joints in the components in different locations of the PCB is approximately the same.     

Reliability studies of two flip-chip BGA packages using power cycling test

Power cycling tests of the second level reliability of two flip-chip BGA packages are discussed in this paper. The first one is for a flip-chip on laminate package (FCPBGA) and the other for a flip-chip on ceramic package (FCCBGA). For the FCPBGA, test strategies will be first discussed and then focus will be given to a unique failure mode associated with this type of packages assembled back to back onto printed circuit board. Instead of anticipated failures of the corner solder joints under the die shadow, as in the case of wire-bonded packages, we found that solder joints failed first in the central region of the package and then failures of solder joints spread out in the radial direction from the center of the package. Explanation will be given to the physical mechanisms that caused this type of failure. For the FCCBGA, the improved test strategies based on what has been learned from the test of FCPBGA will be presented and focus will be given to the effect of different parameters on the second level reliability of the package. Here, because of the increased rigidity of the ceramic substrate solder joints failed as expected first at the corner(s) of the ceramic substrate. Based on the test results and the modified Coffin–Manson equation, predictions or the solder joint fatigue life will be shown.     

Creep deformation behavior in eutectic Sn-Ag solder joints using a novel mapping technique

Creep deformation behavior was measured for 60–100 μm thick solder joints. The solder joints investigated consisted of: (a) non-composite solder joints made with eutectic Sn-Ag solder, and (b) composite solder joints with eutectic Sn-Ag solder containing 20 vol. %, 5 μm diameter in-situ Cu₆Sn₅ intermetallic reinforcements. All creep testing in this study was carried out at room temperature. Qualitative and quantitative assessment of creep deformation was characterized on the solder joints. Creep deformation was analyzed using a novel mapping technique where a geometrical-regular line pattern was etched over the entire solder joint using excimer laser ablation. During creep, the laser-ablation (LA) pattern becomes distorted due to deformation in the solder joint. By imaging the distortion of laser-ablation patterns using the SEM, actual deformation mapping for the entire solder joint is revealed. The technique involves sequential optical/digital imaging of the deformation versus time history during creep. By tracing and recording the deformation of the LA patterns on the solder over intervals of time, local creep data are obtained in many locations in the joint. This analysis enables global and localized creep shear strains and strain rate to be determined.     

陶瓷封装倒装焊器件Sn-Pb微焊点的电迁移行为

高密度陶瓷封装倒装焊器件的焊点尺寸已降低至100μm以下,焊点电流密度达到10~4 A/cm~2以上,由此引发的电迁移失效成为不可忽视的问题。以陶瓷封装菊花链倒装焊器件为研究对象,开展了Sn10Pb90、Sn63Pb37焊点热电环境可靠性评估试验,通过电连接检测及扫描电子显微镜(SEM)等方法对焊点互连情况进行分析。结果表明,Sn63Pb37焊点阴极侧金属间化合物(IMC)增长明显,表现出明显的极化现象,IMC厚度的平方与通电时间呈线性关系。通电时间达到576 h后Sn63Pb37焊点阴极侧产生微裂纹,而Sn10Pb90焊点在通电576 h后仍未出现异常,表现出优异的电迁移可靠性。研究结果对于直径100μm微焊点的陶瓷封装倒装焊器件的应用具有重要的意义。